Analog Circuits and Systems
Prof. K Radhakrishna Rao
Lecture 20: DC Voltage Regulators
1
Review
2
DC Voltage Power Supply
3
AC to DC converter
Full wave rectifier with capacitive filter
Ii ⎛ T ⎞
Peak - to - peak ripple at the output =
⎜
⎟
C⎝2⎠
where Ii is the average current delivered by
AC to DC converter and T = 20 ms
4
Voltage Regulator
5
Characteristics of a Power Supply
Input Power = Pi = VI
i i
Output Power = Po = Vo Io
Efficiency =(η) =
Vo Io
VI
i i
Power dissipated in the voltage regulator
Pi − Po = Pd = VI
i i − Vo Io
; (Vi − Vo )Io as Io ; Ii except for bias current
( V − V ) should be kept to minimum (known as
i
o
dropout voltage) in order to improve the efficiency.
6
Characteristics of a Power Supply (contd.,)
Line regulation : Percentage change in Vo as the input voltage Vi changes
from its maximum to minimum
∂Vo
Vo
x 100
Load regulation : Percentage change in Vo as the output current Io
is changed from its minimum to maximum
∂Vo
Vo
x 100
Temperature coefficient of the output voltage : Change in output voltage
for a small change in input voltage at a specified output voltage
∂V
(
expressed in terms of parts per million =
o
/ ∂T )
Vo
ppm / °C
7
Characteristics of a Power Supply (contd.,)
Output impedance : Change in output voltage for a small
⎛ ∂Vo ⎞
change in output current at a given load current Io ⎜
⎟
⎜ ∂I ⎟
⎝ o ⎠
Ripple rejection factor : Percentage change in output voltage
for a small change in input voltage around the nominal value
⎛ ∂Vo Vi ⎞
of input voltage. ⎜
− PSRR (Power Supply Rejection Ratio)
⎟
⎜ ∂V V ⎟
⎝ i o ⎠
8
Load/Line Transient Regulation
—
—
—
—
Voltage at the output terminal is likely to have undesirable
transients at the time of switching on the input power or when step
load changes occur
The output impedance of the voltage regulator is complex
The presence of feedback loop with large loop gain and the multiple
poles associated with this loop makes the output impedance
complex
Compensation has to be provided to make the output impedance
resistive
9
VCVS as voltage regulator
⎛
R2 ⎞
Vreg = ⎜1 +
⎟ VZ
R1 ⎠
⎝
10
Voltage regulator as a feedback system
VCVS as voltage regulator
⎛
R2 ⎞
Vreg = ⎜1 +
⎟ VZ
R1 ⎠
⎝
11
Voltage regulator as a feedback system (contd.,)
G10
G10 ω1
Forward path G1=
;
for ω1 << ω2
⎛
⎛
s ⎞⎛
s ⎞
s ⎞
⎜1 +
⎟ ⎜1 +
⎟ s ⎜1 +
⎟
ω1 ⎠ ⎝
ω2 ⎠
ω2 ⎠
⎝
⎝
ω2
In internally compensated Op Amps ω1 ;
G10
Hence G1 ;
GB
⎛
s ⎞
s ⎜1 +
⎟
ω2 ⎠
⎝
R1
Feedback path G2 ;
R1 + R 2
12
Voltage regulator as a feedback system (contd.,)
13
Voltage regulator as a feedback system (contd.,)
Vo ⎛
R2 ⎞
; ⎜1 +
⎟
Vi
R1 ⎠
⎝
⎛
⎛
⎛
R2 ⎞
R2 ⎞
R2 ⎞
⎜1 +
⎟
⎜1 +
⎟
⎜1 +
⎟
R1 ⎠
R1 ⎠
R1 ⎠
Vo
⎝
⎝
⎝
=
=
=
Vi
⎛
⎛
⎛
R2 ⎞
R2 ⎞
R2 ⎞
1⎞
2 ⎛
s ⎜1 +
s ⎜1 +
⎜1 + ⎟
⎟
⎟ s ⎜1 +
⎟
gl ⎠
R1 ⎠ ⎛
R1 ⎠
R1 ⎠
s ⎞
⎝
⎝
⎝
⎝
1+
+
⎜1 +
⎟ 1+
GB
ω2 ⎠
GB
ω2GB
⎝
⎛
R2 ⎞
⎜1 +
⎟
R1 ⎠
⎝
=
where Q =
2
s
s
1+
+ 2
ωnQ ωn
GB
⎛
R2 ⎞
⎜1 +
⎟ ω2
R1 ⎠
⎝
14
Voltage regulator as a feedback system (contd.,)
—
—
High rate of rise and least amount of ringing occurs at Q = 1
VZ
R1
is determined by the required output voltage
G2 =
=
Vo
R1 + R 2
—
As w2 and G10 are fixed for a given Op Amp w1 is chosen to make
Q=1
ω2 ⎛
R2 ⎞
ω1 =
⎜1 +
⎟
G10 ⎝
R1 ⎠
—
In internally compensated Op Amps like 741 and TL081 w1 is
already chosen to make Q = 1 for Vo = VZ, i.e., G2 = 1
ω2
ω1 =
G10
15
Voltage regulator as a feedback system (contd.,)
—
—
For any other G2 < 1; Q will always be correspondingly less
than1resulting in unsatisfactory transient response.
It is therefore necessary to use uncompensated Op Amps like
LM748 to design voltage regulators with good transient response.
16
Design of Voltage Regulator using 741/TL081
—
Design voltage regulator with
a reference of 1.2 V for a
regulated output of 10 V for a
maximum load current 15 mA.
17
Design of Voltage Regulator using 741/TL081 (contd.,)
—
Voltage Regulator using Voltage reference IC for 1.2 V: REF3012
⎛
R1 ⎞
⎜1 +
⎟ 10 = 1.2V
R2 ⎠
⎝
R1 = 1.2 kΩ and
R 2 = 8.8 kΩ
The Op Amp used is 741
18
Simulation
19
Simulation
20
Current boosting pass transistor
—
Super Transistor using opamp
21
Current boosting pass transistor (contd.,)
—
using n-channel pass transistor
—
using p-channel pass transistor
- Load Dropout Regulator
(LDO)
22
Voltage Regulator with Current Boosting
23
LM 2940: 1 Amp Low Drop Out Regulator
—
—
—
—
—
—
—
—
—
Input Voltage Range = 6 V to 26 V
Output Current in Excess of 1 A
Dropout Voltage Typically 0.5 V at IOUT = 1 A
Output Voltage Trimmed before Assembly (5, 8, 9, 10, 12 and 15
volts)
Load regulation: 80 to 130 mV from Io = 50 mA to 1amp
Line regulation: 50 to 80 mV for Vi = 2 V to 26 V
Output impedance: 35 to 55 mW at Io = 100 mA Vo = 5 and 8 V
Ripple rejection: 72 to 66 dB for Io = 100 mA Vo = 5 and 8 V
Temperature coefficient of the output voltage: 33 ppm Vo = 5
24
Schematic diagram of LM 2940
It uses a PNP
transistor for current
boosting
25
Using LM2940
* C1 is needed if regulator is located far (more than a few inches) from
power supply filter
** COUT must be at least 22 μF to maintain stability. It may be increased
without bound to maintain regulation during transients. Locate as close as
possible to the regulator. This capacitor must be rated over the same
operating temperature range as the regulator and the ESR (Equivalent
Series Resistance) is critical.
26
Conclusion
27